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Missing building block of life could be made on ice in space

Icy ingredients for life

Dr Keith Wheeler/Science Photo Library

By Joshua Sokol

The search for life in space just got a little sweeter. In the early solar system, ice grains hit by sunlight may have formed sugar molecules on their surfaces, according to a new experiment. Those sugars include ribose: the backbone of RNA, which is implicated in the origin of life.

All known life makes at least some use of RNA as a genetic material, and as the “R” in RNA, ribose holds up the compounds that encode genetic messages. But it’s been hard to understand how ribose could be made in the absence of living organisms, to be part of a precursor for life.

Other components of living cells, such as amino acids, which are the building blocks of proteins, have shown up in experiments and samples from meteorites for years. So have molecules that resemble cell membranes. If they and ribose had all existed at the same time, it could have set the stage for life to evolve.

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But sugars like ribose are hard to come by, since they often stick together in a way that makes them impossible to extract. “Sugars like to react with each other,” says Cornelia Meinert at the University of Nice Sophia Antipolis in France. “In the end, everything is brown like caramel.”

Now Meinert’s team was able to produce ribose by shining ultraviolet light on a frozen blend of water, methanol and ammonia. This mixture represents our solar system in its infancy, before tiny grains of dust and ice collapsed into planets.

Lego castle of life

“It’s another example of how the universe seems to be hardwired to produce a lot of the kinds of compounds you would like to be around if you want to get life going,” says Scott Sandford of NASA Ames Research Center in California. Sandford’s own team is reporting similar results in a paper now in press, he says.

Whether sugars are made on real interstellar ice grains is still an open question. Because these grains are preserved if they gently settle on small bodies far from the sun, checking the surfaces of comets or meteorites may help resolve the issue. ESA’s Rosetta mission and radio astronomers have picked up simple sugars on comets before, but they may struggle to find something complex like ribose, Meinert thinks.

Finding these sugars on comets would tell us that amino acids, molecules in cell membranes and ribose could all have been made in space, then dropped on Earth just in time for the genesis of life.

We’re far from understanding what happened next, though. “Just because now you have all the molecules doesn’t mean you have life,” Meinert says.

Still, it doesn’t hurt. “If you think of all these little molecules we’re making as Lego blocks, and life as a kind of very complex, organised Lego castle, the fact that Lego blocks are falling out of the sky can’t be a bad thing,” Sandford says.